A major problem with colorants is that they tend to fade when exposed to electromagnetic radiation, such as sunlight or artificial light. It is believed that most of the fading of colorants when exposed to light is due to photo-degradation mechanisms. These photo-degradation mechanisms can be attributed to either photo-induced oxidation or reduction of the colorant molecules depending on the environmental conditions in which the colorant is placed. Fading of colorants also depends on the surface chemistry or nature of the type of substrate upon which they reside.
Product analysis of photo-stable products and intermediates has revealed several important modes of photodecomposition. These include electron ejection from the colorant, reaction with ground-state or excited singlet state oxygen, bond cleavage to form various products, reduction to form colorless leuco dyes, and electron or hydrogen atom abstraction to form radical intermediates.
Various factors, such as temperature, humidity, gaseous reactants, including O2, O3, SO2, and NO2, and water soluble, nonvolatile, photo-degradation products have been shown to influence fading of colorants. The factors that effect colorant fading appear to exhibit a certain amount of interdependence. It is due to this complex behavior that observations for the fading of a particular colorant on a particular substrate cannot be applied to other colorants and substrates in general.
An increase in relative humidity of the atmosphere under conditions of constant temperature increases the fading of a colorant for a variety of colorant-substrate systems (e.g., McLaren, K., J. Soc. Dyers Colour, 1956, 72, 527). For example, as the relative humidity of the atmosphere increases a fiber may swell because the moisture content of the fiber increases. This helps diffusion of gaseous reactants through the substrate structure.
The ability of a light source to cause photochemical change in a colorant is also dependent upon the spectral distribution of the light source, in particular the proportion of radiation of wavelengths most effective in causing a change in the colorant and the quantum yield of colorant degradation as a function of wavelength. On the basis of photochemical principles, it would be expected that light of higher energy (i.e., short wavelengths) would be more effective at causing fading than light of lower energy (i.e., long wavelengths). Studies have revealed that is not always the case. Over 100 colorants of different classes were studied and found that, generally, the most unstable were faded more efficiently by visible light, while those of greater lightfastness were degraded mainly by ultraviolet light (McLaren, K., J. Soc. Dyers Colour, 1956, 72, 86).
The influence of a substrate on colorant stability can be extremely important. That is, the kind or nature of substrate material on or in which a colorant is situated can influence colorant stability. Color fading can be either retarded or promoted by some chemical groups within the substrate. For instance, such a group may be a ground-state species or an excited-state species. Porosity of the substrate can also be an important factor in colorant stability. A highly porous substrate can facilitate penetration of moisture and gaseous reactants into the substrate and encourage fading of a colorant. A substrate may also act as a protective agent by screening the colorant molecules from light wavelengths capable of causing degradation.
A need exists for a colorant system, which provides protection to a colorant molecule. A protective encapsulation can shelter sensitive colorant molecules from interaction and degradation caused by such reactants as described above. Encapsulation is a procedure to enclose a molecule, such as a colorant material, in an amorphous solid or semi-solid, such as a polymeric matrix. Persons in the textile dyeing or other associated industries, for instance, would much welcome methods and compositions, which are capable of stabilizing a wide variety of colorants, regardless of the innate stability of the colorant, from the detrimental effects of ambient air and photo-illumination.
A photo-stable colorant can benefit various manufacturers with a means to more easily control precise hues and tints when dyeing or printing, especially when combinations of colors are used. Depending on the particular particle size distribution, the particles can be incorporated as part of many kinds of colored inks for a wide variety of applications. Additionally, functionalization of the particle surfaces can enable one to practice post-treatment or post curing of prefabricated articles and have the particles attached stably to the article substrate. These and other advantages of the present dye particles can simplify and/or standardize currently complicated dye or printing processes.